This application is a national stage of PCT International Application No. PCT/DE2008/001433, filed Sep. 1, 2008, which claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2007 041 669.7, filed Sep. 1, 2007, the entire disclosure of which is herein expressly incorporated by reference.
The invention relates to a method and apparatus for adaptive calculation of pulse compression filter coefficients for a received signal in a radar installation, which received signal is evaluated with the aid of a complex pulse compression mismatch filter wherein a pulse compression filter coefficient set h(t) is calculated for an ideal theoretical received signal s(t) for a pulse compression mismatch filter, such that a pulse compression output signal has a desired main-lobe-to-side-lobe ratio.
German patent document DE 42 30 558 A1 discloses a process for iterative calculation of pulse compression (PC) filter coefficients which is matched in a general form to an ideal theoretical signal (binary code, Barker code, linear frequency modulation, non-linear frequency modulation, polyphase code). The PC filter coefficients calculated using this iteration method have in this case been implemented in a fixed form.
With this method, however, it is not possible to react to signal changes (and therefore to signal deficiencies) which occur during radar operation, or to compensate for them for a high-quality pulse-compression image. These signal changes and signal deficiencies refer to certain reproducible changes to the signal (that is, changes which always recur with the same behavior). A number of options are indicated in the following text for these signal modifications.
1. The PC filter coefficients should be optimized and matched to specific components of the signal processing (that is, for example, to filters that are used). A signal which is passed through an entire signal processing chain in some cases has characteristics that differ from those of a theoretical signal. The PC filter should therefore not be optimized for an ideal theoretical transmission signal, but should be matched (adapted) to a received signal which has been filtered—according to the signal processing.
2. The PC filter should not be optimized for the normally preferred Doppler zero, but for one specific Doppler frequency. By way of example, this can be done for a PC application which takes place at the respective filter outputs after the Doppler processing.
3. The PC filter should be optimized for transmitter deficiencies. These may be caused, for example, by the C-mode operation of the power amplifier. In this case, beyond a specific signal amplitude, the transmitter amplifies completely and the signal enters saturation. In addition, as a result of passing through a transmitter such as this in the C-mode, the signal will in some cases have characteristics that differ from those of a theoretical signal.
4. Generalizing this to generally possible signal modifications: in paragraph 3, it is not sufficient to precisely study the transmitter behavior and then to match the PC filter coefficients to it. In fact, the transmitter behavior is also dependent on the frequency agility of the radar; that is, the transmitter has a different behavior at higher frequencies than at lower frequencies, resulting in significantly noticeable differences in the PC image.
PC filters of a conventional type can be matched to the transmitter behavior only at one frequency. However, if the signal does not change significantly with respect to the PC during a specific time window, then an online calculation of the PC filter coefficients for this time window could significantly optimize the PC image, in an adaptive form.
In its conventional form, in which PC filter coefficients that have been calculated are implemented in a fixed manner throughout the operation of the radar installation, the calculation of the PC filter coefficients is carried out using an iteration algorithm. The iteration process results on the one hand in a corresponding time duration for calculation of the filter coefficients. On the other hand, a certain amount of experience in the use of pulse compression is necessary, in order to allow the desired compressed pulse to be modeled specifically for actual complex-value signals. Effective PC side-lobe suppression can be achieved only if this modeling has been carried out carefully. In consequence it is virtually impossible to implement this process automatically without having to monitor it adequately. This conventional iteration technique is thus unsuitable for adaptive online calculation of the PC filter coefficients.
One object of the present invention is to provide a method which overcomes the above disadvantages of the prior art.
This and other objects and advantages are achieved by the method for setting PC filter coefficients according to the invention, in which such coefficients are adaptively determined for a received signal in a radar installation. To this end, the received signal is evaluated with the aid of a complex pulse compression mismatch filter, and a pulse compression filter coefficient set h(t) is determined for an ideal theoretical received signal s(t) for a pulse compression mismatch filter, such that a pulse compression output signal results with a desired main-lobe-to-side-lobe ratio. A transformed set of pulse compression filter coefficients Hopt(f) for the complex pulse compression mismatch filter Hopt(f) is determined for a distorted received signal using the following rule:
            H      opt        ⁡          (      f      )        =                    S        ⁡                  (          f          )                    ·              H        ⁡                  (          f          )                    ·                        S          v          *                ⁡                  (          f          )                                                                  S            v                    ⁡                      (            f            )                                      2      whereS(f): the Fourier-transform of an undistorted received signal s(t),Sv(f): the Fourier-transform of a distorted received signal sv(t),Sv*(f): the complex conjugate of Sv(f),H(f): the Fourier-transform of the pulse compression mismatch filter h(t).
In the following text, s(t), h(t), Hopt(f) should be understood to be vectors.
A process such as this for optimization and matching (adaptation) of the PC filter coefficients to the given received signal starts from the conventional iteration algorithm for calculation of PC filter coefficients in order to calculate a PC mismatched filter h(t) for an ideal theoretical received signal s(t) (that is, an “uncorrupted” received signal), such that a PC output signal g(t) is achieved with sufficiently good side-lobe separation. In other words: the variables s(t), h(t) and g(t) in the formulas(t)*h(t)=g(t)  (1)are known in the time domain.
This then also applies to the above formula in the frequency domain:S(f)·H(f)=G(f)  (2)where S(f), H(f) and G(f) are the transfer functions of s(t), h(t) and g(t).
According to the invention, an adaptive optimum PC filter hopt(t) which can be calculated online is sought for a received signal sv(t), (which, as ever, is “corrupted”; that is, it includes signal distortions and which may vary during radar operation), such that the PC results in a high-quality PC output signal in the form of a high side-lobe separation. Also, it should be possible to calculate hopt(t) online (that is, quickly and without any monitoring mechanism).
The aim is to produce the same PC output signal (and sufficiently well with respect to the main-lobe-to-side-lobe ratio (MSR)) by means of PC from the “corrupted” (distorted) signal sv(t) and the sought optimum PC filter hopt(t), as in the case of the PC filtering of the “uncorrupted” received signal s(t), therefore g(t).
The following are then obtained based on equations (1) and (2).sv(t)*hopt(t)=g(t)  (3)andSv(f)·Hopt(f)=G(f)  (4)where sv(t) and Sv(f) are known. From the above equations, the transfer function Hopt(f) of the sought optimum PC filter hopt(t) is thus obtained as:
                                          H            opt                    ⁡                      (            f            )                          =                                            S              ⁡                              (                f                )                                      ·                          H              ⁡                              (                f                )                                      ·                                          S                v                *                            ⁡                              (                f                )                                                                                                                        S                  v                                ⁡                                  (                  f                  )                                                                    2                                              (        5        )            wherein Sv*(f) is the complex conjugate vector of Sv(f)), and the impulse response hopt(t) is obtained as the IFFT result (IFFT=Inverse Fast Fourier Transform) of the transfer function Hopt(f).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.